(1) Field of the Invention
This invention relates generally to the correction of aperture shading effects on images and relates more specifically to shading correction on images from digital cameras with more than one aperture plane wherein the said aperture planes are axially distended from one another.
(2) Description of the Prior Art
Photographically, vignetting means the darkening of the corners relative to the centre of an image. All camera lenses suffer, more or less, of these optical phenomena.
A simple lens obeys an optical law called the “cos-4th law” (cosine of the angle of incidence off the lens axis to the 4th power. The cos-4th influence comes from a number of factors, including that the light gets spread out over a larger area, and that the corners of the image are further away from the center of the lens than the on-axis part of the image. This means that for a lens with an angle of view of 20 degrees, where the maximum off-axis angle of incidence is 10 degrees, the corners receive cos (10 degrees) to the 4th power as much light as the center. Cos (10 degrees) is 0.9848, so to the 4th power is 0.9406.
Therefore the corners get 94 percent as much light as the center, a generally imperceptible difference. A 20-degree angle of view for a 35 mm camera equates to a 123 mm lens. For a 28 mm lens, with a 75-degree angle of view, cos 4th comes to 0.39. So now the corners only get 39 percent of the light that the center gets. For a 20 mm lens this goes down to 21 percent, and for a 15 mm lens this goes down to 10.5 percent. Obviously the last case is unworkable and is not acceptable.
In many imaging systems, more than one aperture or pupil may exist in the optical path. Rays that become obstructed by these apertures or pupils do not make it from the object plane to the image plane, and are therefore “shaded” out of the image (this process is also called Vignetting). When one such pupil or aperture plane exists, the shading generally follows a cosine-fourth relationship with ray angle as outlined above. Such shading is generally called lens shading, and is often corrected for in modern digital cameras by a lens shading correction (LSC) algorithm. However, when multiple pupils or apertures exist in the optical path, each axially distended from another, the shading effects become more complicated and additional efforts must be taken to counter these shading effects.
In order to solve said problems with vignetting, cameras are using typically complex optical systems, which are very expensive to manufacture. It is a challenge for the designers of digital cameras to design a camera having very low vignetting at reasonable costs.
Solutions dealing with the suppression of vignetting are described in the following patents:
U.S. patent (U.S. Pat. No. 5,381,174 to de Groot et al.) teaches how for correcting vignetting in the upper or lower range of the pick-up area of opto-electric transducers of a multi-channel color television camera with zoom lens, a representative signal is derived from the diaphragm aperture setting and from the zoom setting. Dependent on corresponding signals for the minimum diaphragm aperture, on the one hand, and the minimum values for wide-angle and tele-setting of the zoom range, on the other hand, these representative signals generate control voltages with which the level of a field frequency saw-tooth signal is controlled.
U.S. patent (U.S. Pat. No. 7,449,802 to Sasaki et al.) discloses an apparatus and method for excluding vignetting occurring during wide-angle shooting using a camera. When a focal length of the camera is computed and an ISO range is set using a result of measuring an amount of light, a view angle wider than the inherent view angle of a camera module can be provided. Therefore, vignetting can be removed and an image of an area wider than the current capture range can be captured. The method for excluding vignetting in a digital camera, comprising the steps of measuring a focal length and an amount of light for an image input from a lens, detecting vignetting and performing an image sensitivity compensation for a vignetting area, and providing a user with an image based on information output from a vignetting processor.
U.S. patent (U.S. Pat. No. 5,012,268 to Hirai) describes an exposure control device for an interchangeable-lens camera in which factors preventing the establishment of a relationship, which ensures proper matching of the photographing lens and the camera body on both the lens, side and the camera body side are taken into account.